#145 "Harness Writing Is Not Hard": Build to Learn, Not to Run with David Soria Parra // Member of Technical Staff @ Anthropic

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David Soria Parra co-created the Model Context Protocol (MCP), the standard that now underpins most AI agent integrations, and, before that, spent years as a core contributor to Mercurial and on Meta's internal source control team. He got into programming at 13, writing a PHP guestbook for a gaming website with friends, and hasn't really stopped building since.

This conversation goes straight to the thing every CTO is quietly building right now: their own agent harness. David makes a surprisingly blunt case that harness writing itself isn't the hard part: "give it a bunch of tools, a bunch of execution steps, be a little smarter about context selection, and go," and that most companies would be better off configuring a strong existing harness than building their own from scratch. He and Tobi also dig into why the "MCP vs. CLI" debate is largely manufactured, how Meta and Anthropic both run on almost no prescribed process, and how David's own workflow has quietly shifted from local Claude Code sessions to Slack threads over the past six months.

CTOs will walk away with a clear-eyed, unhyped view of harness-building from the person who literally created the interoperability standard. Everyone argues about when building your own is worth it, what to check for in a vendor contract, and where to actually put your attention instead of endlessly optimizing your setup.

What's covered:

- David's path from a PHP guestbook at 13 to Mercurial core contributor to Meta's source control team - Biggest lessons from Meta, and how Anthropic runs on even less internal process - His personal setup: a handful of skills, a few MCP connectors, and why he's a "vanilla" tool user - Why his day-to-day workflow moved from Claude Code sessions into Slack threads with Claude - The real MCP-vs-CLI debate — and why it was never actually a debate - Should every CTO or engineer build their own agent harness? - What's still defensible in software engineering as agents get stronger